Field of the Disclosure
The present invention relates to a white organic light-emitting display device, and more particularly, to a white organic light-emitting display device having an n (≥2) stack structure.
Background of the Disclosure
With the increasing interest in information displays and the growing demand for portable information media, a lot of focus is being put on the research and commercialization of thin, lightweight displays.
Among these displays, liquid crystal display devices are a type of display that is getting attention due to their lightweight design and low power consumption.
There is another type of display, i.e., organic light-emitting display devices, which has a wider viewing angle and a higher contrast ratio than the liquid crystal display devices because they are self-luminous. Moreover, the organic light-emitting display devices can be made thinner and lighter and consume less power, because they work without a backlight. Also, the organic light-emitting display devices run on low-voltage direct current, and have fast response time.
Hereinafter, a basic structure and operating characteristics of an organic light-emitting display device will be described in detail with reference to the drawings.
FIG. 1 is a diagram for explaining the principle of electroluminescence of a general organic light-emitting diode.
An organic light-emitting display device generally has an organic light-emitting diode having the structure shown in FIG. 1.
Referring to FIG. 1, the organic light-emitting diode includes an anode 30f as a common electrode, a cathode 30g as a common electrode, and organic layers 30a, 30b, 30c, 30d, and 30e formed between them.
The organic layers 30a, 30b, 30c, 30d, and 30e include a hole transport layer (HTL) 30b, an electron transport layer (ETL) 30d, and an emission layer (EML) 30c interposed between the hole transport layer 30b and the electron transport layer 30d. 
To improve luminous efficiency, a hole injection layer (HIL) 30a is interposed between the anode 30f and the hole transport layer 30b, and an electron injection layer (EIL) 30e is interposed between the cathode 30g and the electron transport layer 30d. 
In the organic light-emitting diode thus constructed, when a positive (+) voltage and a negative (−) voltage are applied to the anode 30f and the cathode 30g, respectively, then holes passed through the hole transport layer 30b and electrons passed through the electron transport layer 30d move to the emission layer 30c to form excitons. When the excitons make a transition from an excited state to the ground state, i.e., stable state, light is emitted at a predetermined wavelength.
In the organic light-emitting display device, sub-pixels are arranged in a matrix, each sub-pixel including the organic light-emitting diode with the above structure. The sub pixels are selectively controlled by a data voltage and a scan voltage, to display an image.
The organic light-emitting display device is categorized as a passive-matrix display or an active-matrix display using a thin film transistor as a switching element. In the active-matrix display, a sub-pixel is selected by selectively turning on its thin film transistor, i.e., active element, and the sub-pixel keeps emitting light by the voltage sustained at the storage capacitor.
Other types of organic light-emitting display include a white organic light-emitting display device that emits white light using at least two emission layers. Since the white organic light-emitting display device emits white light, it has color filters for converting white light to red, green, or blue light.
The organic light-emitting display device that emits white light may have a structure composed of two emission layers of complementary colors. In this structure, when white light passes through the color filters, the range of colors that can be reproduced becomes narrower due to the difference between the wavelength region of the emission peak in each emission layer and the transmission region of the color filters, which makes it difficult to achieve a desired color gamut.
For example, in the case of an organic light-emitting display device that includes a blue emission layer and a yellow-green emission layer and emits white light, white light is emitted while generating emission peak wavelengths at a blue wavelength region and a yellow-green wavelength region. However, when the white light passes through the red, green, and blue color filters, the transmittance of the blue wavelength region becomes lower than those of the red or green wavelength regions, resulting in a decrease in luminous efficiency and color gamut. Also, the blue emission layer is formed of a fluorescent light-emitting material, and the yellow emission layer is formed of a phosphorescent light-emitting material, and this leads to a difference in efficiency between the yellow phosphorescent emission layer and the blue fluorescent emission layer because the yellow phosphorescent layer has a higher luminous efficiency than the blue fluorescent emission layer, resulting in a decrease in luminous efficiency and color gamut.